The present invention relates to a magnetic recording and reproducing apparatus using video heads with less turns of the winding.
One example of magnetic recording and reproducing apparatus is a household video tape recorder (VTR). The winding of the video head used in such VTRs needs 15 turns to 20 turns, and such large number of turns adversely affects the productivity of video heads. FIG. 1 shows an example of the structure of the video head. As can be seen from the figure, the winding of a video head 1 is arranged by threading a copper wire 2 for 20 times through an aperture 3, which is similar to the hole of a pin, provided in the head. This fabricating phase is not only a time-consuming work, but also a cumbersome job for workers. The following will describe the reason why the video head in the conventional VTR employs a winding of 20 turns.
FIG. 2 is a block diagram showing briefly the circuit arrangement of the conventional 2-head helical scanning VTR.
The recording operation will first be described. The video signal received at the input terminal 12 is fed through the recording circuit 10 and converted into the recording signal. The recording signal is amplified by the writing amplifier 7, which then provides the recording current in the video heads 1a and 1b. Each of the video heads 1 has a winding whose inductance is chosen to be around 2 .mu.H in 20 turns. The rotary transformers 5a and 5b have a turn ratio (the ratio of the number of turns of the secondary winding to the number of turns of the primary winding) of 1:2. The primary windings (head side) of 5a and 5b have four turns and three turns, respectively, and the secondary windings (head amplifier side) of 5a and 5b have eight turns and six turns, respectively.
In the above conditions, the writing amplifier 7 is required to produce an output signal voltage (peak to peak) of 3-4 V P-P for providing the recording current for the oxide film tape. The magnitude of 3-4 V P-P is appropriate for the power voltage of 9-12 V for the writing amplifier 7. If the head has a lower impedance seen from the writing amplifier than that mentioned above by provision of less number of turns for the heads 1a and 1b or different turn ratio for the rotary transformers 5a and 5b, the recording current will increase, resulting disadvantageously in an increased power dissipation. On the other hand, if the head has a higher impedance, a higher output signal voltage will be required for the writing amplifier, causing a distortion in the signal waveform.
Next, the picture reproducing operation will be described. The signals produced by the video heads 1a and 1b are conducted through the rotary step-up transformers 5a and 5b, then fed to the head amplifiers 8a and 8b. Capacitors 9a and 9b are provided to have a resonant relationship with the inductances of the heads seen from the head amplifiers 8a and 8b, respectively, with each resonant frequency being chosen in the neighborhood of the maximum carrier frequency of the reproduced FM signal. Such resonant frequency is effective for suppressing impedance noises and also for noise matching between the head amplifiers 8a and 8b and the video heads 1a and 1b.
Accordingly, if the signal source impedance is lowered by reducing the turns of the head or by changing the turn ratio of the rotary transformer, the above-mentioned resonant characteristics may be retained, but the noise matching between the heads 1a and 1b and respective amplifiers 8a and 8b will be deteriorated. Conversely, a higher signal source impedance makes noise matching easy, however, the resonant characteristics will be lost. The resonant frequency is determined by the inductance of the head and the total capacitance of the stray capacitance, input capacitance of the amplifier and capacitors 9a and 9b. Therefore, if the inductance of the head is too large, the required high resonant frequency cannot be obtained even with the smallest value for the capacitors 9a and 9b.
The following will explain the reason why the rotary transformers 5a and 5b have different number of turns. FIGS. 3A and 3B are the plan view and cross-sectional view of the rotary transformers 5 shown in FIG. 2. The rotary transformer used in household VTRs has the structure as shown in FIGS. 3A and 3B, where a rotor core 14 and a stator core 15 confront each other with windings 16 and 18 for the first channel and windings 17 and 19 for the second channel being disposed concentrically.
The question here is the different diameters of the winding 16 for the first channel and the winding 17 for the second channel. If the windings 16 and 17 have the same number of turns, the inductance of the primary winding for the second channel having a larger diameter will be larger than that for the first channel, causing the unevenness among the channels. The difference of diameter is compensated by providing different number of turns for each channel, such as, for example, four turns for the first channel winding 16 and three turns for the second channel winding. Variable resistors 14a and 14b are provided for adjusting the boosting value at the resonant frequency, however, they cause a deterioration of the S/N ratio.
In FIG. 2, the output signals from the amplifiers 8a and 8b are fed to the reproduction signal processing circuit 11, where the signals are demodulated into the video signal, and the reproduced video signal is obtained at the output terminal 13.
Next, the relationship between the inductance of the primary winding of the rotary transformer 5 and the inductance of the video head will be described. If the rotary transformer 5 is an ideal transformer with a coupling coefficient of one, it is not necessary to provide a specific relationship between the inductance of the rotary transformer and that of the head. However, the rotary transformer 5 has an air gap 20 as shown in FIG. 3B, causing the coupling coefficient (K) to fall to a value around 0.95. The matching condition for the inductance (Lh) of the video heads 1a and 1b and the primary winding inductance (L.sub.RF) of the rotary transformer with the coupling coefficient (K) being unequal to one is expressed as follows. ##EQU1##
The above condition makes the value of e.sub.o /.sqroot.L.sub.o maximum for picture reproduction, where L.sub.o is the inductance of the head seen from the secondary winding of the rotary transformer 5 and e.sub.o is the no-load output. Accordingly, the value of e.sub.o /.sqroot.L.sub.o indicates the figure of merit for the frequency band of the reproduced FM signal (3-5 MHz in the case of VHS system). On the other hand, the condition that L.sub.RF becomes larger than Lh/.sqroot.1-K.sup.2 will result from an increased value of Lo with e.sub.o being constant, causing disadvantageously the resonant frequency to fall and the impedance noise to slise. Accordingly, the condition that L.sub.RF is larger than Lh/.sqroot.1-K.sup.2 to the extent where the resonant frequency is obtained is also acceptable for a system where the noise caused by the tape is larger than the impedance noise.
As regards the reading out of the chroma signal which is multiplexed in the frequency band below the FM signal band, the signal source impedance for this frequency band is very small and can be neglected. Accordingly, the figure of merit for the chroma signal is e.sub.o, and the matching condition becomes simply L.sub.RF &gt;&gt;Lh.
Consequently, the values of L.sub.RF and Lh for the existing household VTRs should preferably be chosen in accordance with the following equation. ##EQU2## where .gamma. ranges from 1.0 to 1.5.
If the value of .gamma. is too large, the resonant frequency determined from the inductance Lo, the stray capacitance, the capacitors 9a and 9b, and the input capacitance of the amplifiers 8a and 8b will become too low to obtain the reproduction FM equalizing characteristics. Accordingly, the value of .gamma. should be chosen so that the above-mentioned resonant frequency becomes higher than the reproduced FM carrier frequency.
In the conventional VTRs, however, there has been made no efforts to reduce the turns of the head, and neither having been paid attention for choosing a small L.sub.RF, a large K and a large .gamma..
Under the foregoing situation, the conventional VTRs employ video heads having windings of 20 turns (Lh=2 .mu.H), rotary transformers having primary windings for the first and second channels of 3 turns and 4 turns, respectively, (L.sub.RF =10-12 .mu.H, K=0.97) with a turn ratio of 1:2, and head amplifiers providing a recording signal voltage of 3-4 V P-P with a noise matching impedance of 200 .OMEGA. to 1 k.OMEGA..
The prior art deficiencies are summerized as follows. (1) The rotary transformer has an inner winding of four turns and an outer winding of three turns. (2) In order for the video head to match the rotary transformer, the video head needs to have a winding of 20 turns. (3) The rotary transformer has a turn ratio of 1:2 so that the recording signal level is not too high and the noise figure of the head amplifier is not deteriorated under the above-mentioned conditions.